The present invention relates generally to signal detection, and more specifically to apparatus and techniques employing quadrature correlation for precisely detecting coded tone pulses characterized by a known duration and carrier frequency and having a carrier phase reversal at a predetermined time during each pulse.
Tone pulses of a known carrier frequency, width and other characteristics are employed in a variety of situations for communication, detecting and locating objects and/or sound sources, and for other purposes. To the extent that the tone pulses remain relatively undistorted and of significantly higher amplitude than any background noise, pulse detection can be accomplished with relative ease and simplicity. However, there are many situations in which it is necessary to reliably and precisely detect tone pulses in a signal contaminated with noise of significant amplitude relative to the tone pulse amplitude. Conditions adverse to reliable pulse detection may, for example, occur because of substantial attenuation of the tone pulses over the signal transmission path or because of a high level of ambient noise from any of a variety of sources. Under such conditions simple amplitude and/or frequency discrimination may not provide acceptable tone pulse detection.
In certain applications it is particularly important that the pulse arrival time be determinable with a high degree of accuracy. It has been common to use leading edge detection to determine pulse arrival time. However, this method is subject to significant errors which are a function of signal level relative to detection threshold. It is also known to use pulse differentiation in determining arrival time. According to this technique, a pulse is heavily filtered so as to produce a rounded pulse envelope. The corresponding differentiation function is characterized by a zero crossing at the envelope peak. This method suffers from sensitivity to pulse distortion, which causes the peak to shift relative to the true pulse center. Accordingly, neither leading edge detection nor pulse differentiation techniques are adequate for some applications.
One particular field in which precise pulse detection has recently been of particular concern involves acoustic systems for accurately indicating marine vessel position. There is increasing interest in exploring for oil and minerals on or beneath the ocean beds in deep water, producing oil and minerals from such locations, conducting marine research in very deep water and maintaining and servicing equipment used in connection with the foregoing activities. Such activities require the ability to rapidly and accurately ascertain marine vessel position relative to a location of interest. Precise position information is also demanded in other specific applications in which it is desired to maintain fixed vessel position, or to maneuver a vessel between locations along a predetermined route in accordance with sensed parameters. Further, it is required or highly desirable to obtain this information without physical connection to the ocean bed or object of interest.
Acoustic systems have been found to possess superior characteristics for marine vessel position sensing applications. One successful type of acoustic position indicating system comprises a transponder or beacon located in a known positional relationship with an underwater point of interest, an array of acoustic signal receiving elements or hydrophones located on the vessel, means for sensing phase and/or time differences between transponder signals received at pairs of hydrophone elements and computation apparatus for determining the location of the hydrophone array relative to the transponder from the phase and/or time differences.
The hydrophone array comprises a minimum of three hydrophone elements. Two elements are located along each of two generally horizontal transverse axes, one element being common to both axes. In one specific system, the phase of the signals received by each of the hydrophone elements relative to a reference signal is periodically determined, and the phase information supplied as a succession of digital signals. Such a digital phase determining system is disclosed in detail in U.S. Pat. No. 4,038,540 for a Quadrature Correlation Pulse Detector issued July 26, 1977 in the name of J. L. Roberts and U.S. Pat. No. 4,071,821 for Quadrature Correlation Phase Determining Apparatus issued Jan. 31, 1978 in the names of W. P. Harthill and J. L. Roberts, both patents being assigned to the same assignee as the present application. Angular displacement of the vessel from the transponder in transverse vertical planes is computed from differences in the phases determined for the signals received by the hydrophone elements.
Although the previously described system has been found to provide performance superior to other types of systems, the quality of position indications produced thereby is generally dependant on acoustic paths having constant transmission characteristics between the transponder and hydrophone array. The subsea environment typically does not provide ideal acoustic transmission paths. Factors resulting in variable transmission characteristics include reflection and/or refraction of signals from thermal layers in the water, scattering of signals from water borne particles and reflection of signals from underwater structures. In addition, acoustic signals emanating from sources other than the beacon of interest may cause spurious phase determinations. These factors produce a severe acoustic signal transmission environment having effects on signal transmission which are not presently susceptible of complete analysis. As a result, the apparent arrival times for the signals received by the hydrophone elements, the corresponding time and/or phase differences, and ultimately, the indicated vessel position may deviate excessively from an accurate position representation.
One known technique for improving the timing accuracy of pulse detection involves the use of a coded tone pulse characterized by a known duration and carrier frequency, and having a carrier phase reversal during the pulse. Such a technique is discussed in Dixon, Spread Spectrum Systems, John Wiley and Sons, Inc. (1976), chapters 2 and 5. The technique is also embodied in pulse detectors disclosed in U.S. Pat. Nos, 4,004,235 and 4,013,967 issued respectively to J. L. Roberts and F. A. Fassbind on January 18 and March 22, 1977.
Another known general technique for improving signal detection involves correlation of two signals, one of which may be a reference signal. Both analog and digital forms of signal correlation are known. For example U.S. Pat. No. 3,346,862 issued to I. G. Raudsep on Oct. 10, 1967 discloses an analog autocorrelation system for determining the time difference between a pair of pulse signals of common origin. The system employs weighting filter means for modifying the power spectra of the pulsed signals to optimize the autocorrelation function. U.S. Pat. No. 3,646,334 issued to I. Wold on Feb. 29, 1972 discloses a hybrid analog/digital digital system in which two input signals to be correlated are sampled, the samples of one of the signals inserted into a recirculating memory time compressor, the output of the memory multiplied with the other signal, and the product signal averaged to determine correlation of the input signals.
Other known refinements in correlation techniques involve multiplication of the input signal with each of quadrature components of a reference signal. The product signals are integrated with respect to time to produce real and imaginary components of correlation of the input and reference signals. The real and imaginary components are combined in accordance with the Pythagorean theorem to produce an indication of correlation of the signals. An analog variation of this method is embodied in a signal processor disclosed in U.S. Pat. No. 3,878,526 issued to N. E. Pederson on Apr. 15, 1975. A digitally implemented signal detector employing elements of the method is disclosed in U.S. Pat. No. 3,925,732 issued to H. Tanaka et al on Dec. 9, 1975. The latter implementation is based on the premise that, for the application being considered, the polarity of a band limited signal contains nearly as much information as an analog signal from which it was derived. Accordingly, an analog input signal may be represented by a time series of two possible voltage states. This so called "clipped signal" may be simply and conveniently produced by a clipper or clipping amplifier.
The applicant has discovered that the advantages of mid-pulse carrier phase reversal detection, quadrature correlation and digital signal processing utilizing clipped signals may be uniquely combined to provide a simple but precise pulse detector and detection technique. This detector and detection technique have otherwise been found to possess characteristics and features which are particularly advantageous in making time and phase determinations of the quality required in marine vessel acoustic position indicating systems.
It should be noted that although the pulse detection apparatus and technique disclosed herein have been found particularly useful in marine vessel position indicating systems, the concept is of general utility. It may be implemented in a variety of ways to meet the requirements of a broad range of applications requiring precise but reasonably simple tone pulse detection.